The present invention relates to an optical disc discriminating system in an optical disc player for discriminating various kinds of optical discs such as CD and DVD (Digital Video Disc or Digital Versatile Disc).
A compatible single optical disc player which is capable of reproducing a CD (compact disc) and a LD (laser disc) has been popular. Since the thickness of the CD and the LD from the rear surface to the recording surface is the same (1.2 mm), it is possible to use an optical pickup in which a laser beam is focused on a focal point for reproducing both of the CD and the LD.
In order to discriminate the kind of the disc mounted on the player, the diameter of the disc is detected for discriminating the CD or the video disc.
Recently, the DVD having a high recording density has been developed. Although the thickness of the CD from the surface to the recording surface is 1.2 mm, the thickness of the DVD is about 0.6 mm. However, the principle of a reading system of recording pits of the DVD is the same as that of the CD. Therefore, a compatible single optical disc player which is capable of reproducing the CD and the DVD has greatly been proposed.
In such a compatible CD/DVD player, in order to properly focus the beam for reading the information on the recording surface of the disc, two types of lenses are provided, each having a focal length suitable for a thickness of a corresponding disc. As another type of the compatible CD/DVD player, a double focus lens using a hologram element is provided.
FIG. 8a shows an optical pickup having a double focus lens using a hologram element (diffraction grating).
The double focus lens comprises a diffraction grating 13, an objective lens 11 disposed on the same optical path, and a collimator lens 14. A beam of light is paralleled by the collimator lens 14 and divided into three spectral beams of 0 order spectrum (having a 90 diffraction angle) and .+-.1 order spectrum by the diffraction grating 13 (-1 order spectrum is not shown). Since focal lengths of the 0 order spectrum and the +1 order spectrum are different from each other, each of the 0 order spectrum (hereinafter called zero-order spectrum) and the +1 order spectrum (hereinafter called first-order spectrum) is focused on a different position of the same line.
Concretely, the zero-order spectrum is properly focused on the recording surface of the DVD, while the first-order spectrum is properly focused on the recording surface of the CD.
In operation, when the pickup is gradually moved away from the disc, the recording surface is irradiated with the zero-order spectrum first. A four-divided photodetector provided in the pickup (not shown) produces an S-shaped signal as a focus error signal. When the pickup is further moved away, the first-order spectrum reflected from the disc passes an optical path of the zero-order spectrum, thereby forming a pseudo spectrum. Thus, a pseudo S-shaped signal of the pseudo spectrum is produced. Finally, an S-shaped signal of the first-order spectrum is produced as a focus error signal.
FIG. 8b shows S shaped signals of the corresponding zero-order spectrum, pseudo spectrum, and first-order spectrum with respect to the movements of the double focus lens of FIG. 8a. The spectral ratio of the zero-order spectrum and the first-order spectrum is set to 1:1.
As aforementioned, the zero-order spectrum achieves the optimum focus on the DVD, while the first-order spectrum achieves the optimum focus on the CD. If the amount of the S-shaped signal by the zero-order spectrum on the DVD is 100%, the amount of the signal corresponding to the CD is small, less than 100%. Similarly, if the amount of the S-shaped signal by the first-order spectrum on the CD is 100%, the amount of the signal corresponding to the DVD is small, less than 100%. Thus, in the double focus lens, the amplitudes of the respective focus error signals are different from each other. The ratio of zero-order spectrum to first-order spectrum of the DVD, namely zero-order spectrum/first-order spectrum is larger than the ratio of zero-order spectrum to first-order spectrum of the CD, that is zero-order spectrum/first-order spectrum.
In the compatible CD/DVD player, a focus servo control system is provided for each disc. In a focus servo control, the pickup is vertically moved against the disc, and a focus error (FE) signal is produced for detecting a proper focal point for focusing in. However, in the aforementioned player using the double focus lens, a plurality of focus error signals (S-shaped signals by zero-order spectrum, pseudo spectrum and first-order spectrum) are obtained. Accordingly, a problem that focusing in is achieved by an S-shaped signal detected first arises. In other words, there may be occur an erroneous focusing by an improper spectrum.